This invention relates to a high quality amplifier circuit for telecommunication equipment, more particularly to a multistage amplifier circuit providing a linear input-output characteristic for an input signal having a level which varies over a wide range.
When a multistage amplifier circuit is used in a circuit in which the input signal level varies widely, its output level must be linear over a range which extends up to the highest possible level of the input signal. In addition, an amplifier circuit used in transmission equipment is generally required to have low noise characteristics for the lowest level of the input signal. A circuit of low cost and small size is of course desirable. A typical application for such a multistage amplifier circuit is a microwave receiver in a radio relay system.
A microwave receiver is generally composed of a first stage amplifier for the radio frequency signal, an interstage amplifier for the intermediate frequency signal and a last stage amplifier for the final intermediate frequency signal. FIG. 1 is a block diagram of a multistage amplifier circuit which includes a first stage amplifier 1, an interstage circuit 2 and a last stage amplifier 3. The first stage amplifier 1 is a radio frequency amplifier (RFA). the interstage circuit 2 consists of a frequency mixer (MIX) 21, a local oscillator (LOSC) 22 and an intermediate frequency pre-amplifier (IFA) 23. The last stage amplifier 3 is a main intermediate frequency amplifier (MIFA).
A radio frequency signal P.sub.1 from a receiving antenna (not shown) is applied to the radio frequency amplifier (RFA) 1. The output signal P.sub.2 of the radio frequency amplifier 1 is mixed with the output of the local oscillator (LOSC) 22 in the frequency mixer (MIX) 21. In the frequency mixer 21, the input signal P.sub.2 with frequency f.sub.r is converted into an intermediate frequency signal with frequency f.sub.if. Frequency f.sub.if is the frequency difference between the input signal frequency f.sub.r and the local oscillator frequency f.sub.loc, and frequency f.sub.if is often selected to be 70 MHz.
The input level of the input signal P.sub.1 often fluctuates in a wide range from its standard level, a phenomenon which is called "fading". Fading is caused by a change in meteorological conditions along the propagation path of the radio wave. The multistage amplifier in a microwave receiver is required to have a linear input-output characteristic up to the highest level of the input signal P.sub.1 caused by fading, as well as to have low noise at the lowest level of the input signal P.sub.1.
In the conventional receiver circuit of FIG. 1, the first stage circuit (RFA) 1 of the multistage amplifier circuit is required to be a low noise and high gain amplifier to provide an overall low noise receiver. Thus, when the input signal of RFA 1 attains a high level due to fading, the output signals of the frequency mixer (MIX) 21 and pre-amplifier (IFA) 23 will be saturated or distorted. In order to avoid saturation of the signal in the mixer 21 or pre-amplifier 23 of a conventional multistage amplifier, it has been necessary to use expensive circuitry and components.